The present invention relates to an integrated semiconductor optic sensor device having a desired spectral selectivity and obtained by a standard CMOS process.
One of the most important fields of application for these types of devices is for integrated color sensitive photosensors, for instance for color images or a color television camera.
Optical light sensors based on semiconductor properties are widely used for several video image applications. Various techniques may be used for optical to electrical conversion. One of the most effective is based on electron-hole generation due to the light absorption of a semiconductor reverse biased photodiode.
Since the final effect of the electron-hole generation doesn""t represent the wavelength of the absorbed light in the optical range, this physical mechanism cannot be directly used to distinguish different colors.
To implement color sensitivity, a series of colored filters is generally provided between the light source and the photosensitive device. This is usually implemented by a deposition of an organic colored resin over the finished semiconductor photosensitive device. This resin stops by absorption the unwanted colors of the incident light and transmits to the light sensor only the light wavelengths to be selected. In this manner the electric signal generated in the semiconductor device is correlated to the desired color only.
In the case of color selectivity used for CMOS video cameras, the photodiodes are integrated on silicon to form a bidimensional matrix. From the top view each diode looks squared. Each diode is electrically insulated from the other adjacent diodes by an isolation region, for instance a field oxide.
To clearly detect color images, the semiconductor matrix includes at least three different kind of staggered diodes, which are sensitive to blue, green and red light respectively.
The main drawbacks of the photodiodes covered by the organic resin are that the resin deposition requires an additional process step once the diodes are formed, and that the filter thus created absorbs a portion of the incident light, thereby reducing the diode sensitivity.
Moreover, long exposure to intense light, humidity, high temperature and ionizing radiation may reduce the ability of the organic resins to block the unwanted colors.
A prior art solution for providing color selectivity is disclosed in the European patent No. 0152353, which relates to method and device for obtaining a color selecting effect based on the wave properties of the light. This method may be implemented during the semiconductor device manufacturing process instead of on the finished sensor.
Briefly, this method includes depositing a stack of inorganic layers over the light sensor device. These layers have suitable thicknesses and indexes of refraction.
By a suitable definition of the stack structure, it is possible to obtain the desired spectral transmission of the incident light toward the semiconductor sensor device.
By modulating the interference of the light waves reflected at all the layer interfaces of the stack it is possible to maximize or minimize the light intensity transmitted or reflected by the entire stack in pre-determined wavelength ranges, that is, colors.
The method disclosed in the above cited European patent is based on the construction of an optical resonant semiconductor stack formed by the following layers: Monosilicon-Oxide-Polysilicon.
The incident light is reflected by the interface of the monosilicon/oxide and interferes with the light reflected by the other interface of oxide/polysilicon.
The above disclosed method of wavelength selection by interference creates a more reliable sensor than the sensor covered by conventional organic resins, because the conventional organic resins have a tendency to breakdown over time, as discussed above. This better reliability is appreciated especially in those application fields where high temperature or humidity is present, or in cases of high light exposure of the sensor, or even in presence of ionizing radiations.
Moreover, the interferential technique can be integrated in standard VLSI semiconductor processes without the need of a dedicated and separated process and environment to apply color selecting resins on the finished sensor.
As explained above, interferential stacks are formed by materials having a high and a low refraction index, respectively. Polysilicon, or amorphous silicon, is generally used as high refraction index material, while silicon dioxide is commonly used as low index material.
The use of Polysilicon leads to an intrinsic parasitic absorption of light that involves a consequent loss of a portion of the optical signal before it reaches the sensor.
This fact limits the use of the interferential method to the construction of simple resonators with one or very few layers of a very thin high index material, for instance few hundreds of Angstrom. This is a severe limitation on the large variety of possible transmission spectra theoretically obtained with the interferential method.
Until now, no device has been produced that can preserve the interferential properties of the integrated stack sensors, while also avoiding the excessive absorption of light within the interferential stack.
Embodiments of the present invention use a high refraction index layer having the highest possible transparency, for instance a transparent metallic oxide having a high dielectric constant, in order to improve the current interferential applications. These improvements, included in applications such as video cameras, greatly widen the field of application of integrated interferential photodetectors.
Embodiments also improve the spectral sensitivity of the photodetectors that is not available with the standard colored resins or with interferential integrated stacks of known type that make use of the polysilicon layer.
Embodiments of the invention also make the design of resonators more flexible by increasing the complexity of interferential stacks by including any number of high refraction index layers and low refraction index layers, thereby achieving a wide range of spectral transmission curves.
Presented is a device and a process for manufacturing a light sensor device in a standard CMOS process. During fabrication, the active areas on a semiconductor substrate are implanted to obtain at least a first integrated region of a corresponding photosensor. Then a stack of layers having different thickness and refractive index over the photosensor are formed to provide an interferential filter for the photosensor. At least one of the layers in the stack is formed by a transparent metallic oxide having a high refraction index.
In this manner, the design of interferential resonators is rendered more flexible because a stack of layers including more than one high refraction index layer is possible. This possibility wasn""t previously currently available since non totally transparent high index materials, such as polysilicon, are currently used according to the teaching of the prior art solutions.
The features and advantages of the invention will become apparent from the following description of an embodiment thereof, given by way of non-limiting example with reference to the accompanying drawings.